NEW INSIGHTS INTO MISSISSIPPIAN PALEOGEOGRAPHY IN CENTRAL TENNESSEE: SHALE-FILLED CHANNELS ON A CARBONATE SLOPE

Shale-filled channels in the rock record pose an interesting paradox of mud deposition in an open conduit. Recent flume studies have shown that clay- and silt-sized particles are easily erodible, even after sustained periods of drying. It is a logical assumption that mud deposition within a channel would be temporary, whereas mud deposition in overbank areas or distal reaches of a submarine system would be continuous.

Highway construction in north-central Tennessee has exposed 3.5 km of outcrops characterized by broad, sharp-based channel features, which incise organic-rich mudstones of the Fort Payne Formation (Middle Mississippian). The channels are filled with alternating layers of shale and crinoidal grainstones. Imbrication of crinoid stems, normal and reverse grading of crinoid fragments and traction carpets of larger carbonate allochems suggest the channels were conduits for flow in an ancient shallow marine system. Thus, the occurrence of thick mudstone beds (cm- to m-scale) within channel features is problematic. If flow velocity was enough to transport allochems, why did it not erode clay and silt-sized particles? Co-deposition of this nature has not been addressed in previous studies.

The sedimentary source(s) and paleotransport direction(s) for the channels are presently unknown. Paleogeographic reconstructions of the study area during the Middle Mississippian place the clastic Borden Delta to the northeast, the Fort Payne carbonate ramp to the southeast, and an extensive, long-lived carbonate platform to the west. A detailed analysis and 3D reconstruction of channel systems is needed to constrain which of these contributed sediment to the shallow marine system.

This study combines preliminary field mapping, thin section analysis, and scanning electron microscopy (SEM) to interpret: (1) sedimentological processes responsible for co-deposition of siliciclastic mudstones and crinoidal grainstones; (2) sediment transport directions; (3) implications for regional channel trends; and (4) porosity and permeability variation in channel features. With respect to the latter, Mississippian rocks in central Tennessee are exploration targets for oil and gas development.